Osmotic blistering is a chemical phenomenon where two substances attempt to reach equilibrium through a

semi-permeable membrane Semipermeable membrane is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by osmosis. The rate of passage depends on the pressure, concentration, and temperature of the molecule ...

Overview

Water will flow from one solution to another, trying to create equilibrium between both solutions. Usually, the two solutions are

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...

and the coating application on top of the concrete. Concrete is very porous, so water beneath the concrete, will force itself through the concrete, typically through vapor transmission. The water will then try to break through the semi-permeable membrane (either the surface of the concrete or the primer). Most

epoxies The Epoxies were an American New wave music, new wave band from Portland, Oregon, formed in 2000. Heavily influenced by new wave, the band jokingly described themselves as robot garage rock. Members included FM Static on synthesizers, guitarist ...

or urethanes or

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

applications are not permeable so the water will stop at the polymer coating. However, the pressure from the water does not stop, forcing the water to collect directly in between the concrete and the layer of epoxy/urethane. This collection creates the notorious “osmotic blister” that is commonly feared by coating specialists. For steel substrates: The presence of soluble salts (particularly sulfates and chlorides) at the metal/paint interface is known to have a detrimental effect on the integrity of most paint systems including fluorocoatings. The salts come from atmospheric pollution and contamination during blasting or other substrate preparation processes. These salts promote osmotic blistering of the coating and underfilm metallic corrosion. As a result, loss of adhesion, cathodic disbondment, and scribe creep can be observed. A coating behaves as a semi-impermeable membrane; allowing moisture to pass through but not salts4,5. When a paint coating is applied on a metallic surface contaminated with soluble salts, an osmotic blistering process takes place (Figure 8.10). Osmosis is the spontaneous net movement of solvent molecules (water) through a semipermeable membrane (coating film) into a region of higher solute concentration (the salt contaminated substrate). The process drives to equalize the solute concentrations on the two sides, but because salt cannot pass through the membrane (coating) it can never equalize. Water continues to permeate into the region. As the soluble substance dissolves under the paint layer, the pressure caused by the increase in volume can exert a greater force than the paint adhesion and cohesion forces, giving rise to the formation of a blister; the process called osmotic blistering. The blisters are first filled with water and later with corrosion products from the corrosion of the metallic substrate.

References

{{Reflist Concrete Membrane technology

Overview

See also

References